1. Field of the Invention
The present invention relates to a front-end of a wireless communication receiver and, more particularly, to a wide-band intermediate frequency (IF) radio receiver and a direct conversion radio receiver.
2. Background of the Related Art
The radio frequency (RF) front-end design in a wireless transceiver is perhaps the most important part of the receiver's design, because its performance in terms of the noise figure and linearity determines the overall performance of the receiver. There exist a number of architectures for implementing the radio transceiver. Two common ones are the heterodyne architecture and the homodyne or direct conversion architecture.
As the demand for multi-mode and multi-band radio receivers grows, so too does the attention given to the direct conversion radio receiver. Although it has a simply illustrated configuration, practical use of the direct conversion receiver in the wireless market has been delayed due to its inherent problems. These problems relate to a direct current (D/C) component offset noise, which is proportional to the inverse value of the frequency (1/f), and even-order distortion. Of these, the DC offset problem is more difficult to solve, because there is a conflicting condition between DC offset measurement and DC offset correction. The main cause of the DC offset results from carrier leakage.
FIG. 1 illustrates a block diagram of the related art direct conversion receiver that suffers carrier leakage. Spectral leakage 5 or coupling of the local oscillator (LO) 1 signal is added to the input signal of the low-noise amplifier (LNA) 2. This added signal has the same frequency component as the LO 1 signal, and thus, produces the DC component at the mixer 3 output. This DC component, in turn, is added to the incoming signal. In a typical wireless application of the related art, the required dynamic range exceeds 80 decibels (dB), and thus, the output of the receiver can be saturated even with the small offset.
Since the DC offset is located very close to DC, the measurement process of the DC offset requires too much time to perform for an offset cancellation technique based on averaging. Also, the correction time for cancelling the DC offset tends to be long. In wireless systems requiring a fast DC offset cancellation, most approaches are not feasible because of the above-described reasons. Therefore, reducing the DC offset to a negligible level, rather than completely eliminating it, may be highly desired. To reduce the DC offset, it is quite important to reduce the amount of carrier leakage to the antenna 4 and to the input of the LNA 2.
FIG. 2 illustrates a related art wide-band zero IF, RF front-end design, used in recent years, that reduces the carrier leakage. A complete description of FIG. 2 may be found in U.S. Pat. No. 5,761,615, which is hereby incorporated by reference. The illustrated configuration is similar to a related art heterodyne receiver. However, the main difference between the related art heterodyne receiver and the illustrated design is the selection of the intermediate frequency (IF).
FIG. 3 illustrates a related art super-heterodyne receiver. For the super-heterodyne receiver, the IF frequency is chosen to be about one-fifth the carrier frequency, to solve the trade-off between image and channel selectivity. Before the down conversion 31, an image rejection filter 32 is inserted to block the signal of the image band. After first down conversion 31, the narrow band-pass filter 33 rejects the unwanted signal.
The principal difference between the designs of FIGS. 2 and 3 is the absence of the band-pass filter, used for channel selection, and of the image rejection filter in the design illustrated by FIG. 2. This is feasible in the circuit of FIG. 2, because the IF frequency is so high that the front-end filter, intended to provide band selection, is also sufficient to reject the image signal. Channel selection is done at baseband, instead of at IF band. When using high performance baseband circuits, the performance degradation of wide-band IF conversion, such as that performed by the circuit illustrated in FIG. 2, becomes negligible.
In the wide-band zero IF receiver, the sum of the two local oscillator frequencies 21 and 22 is equal to the original carrier frequency 23. Whenever signal amplification is done at baseband, careful design of the RF front-end is required to produce a low DC offset. This is because alternating current (AC) coupling or its equivalent is not as pronounced, due to its long settling time constant. The most critical source for the DC offset is the carrier leakage.
Referring now to FIG. 1, the carrier leakage is inevitable when there are sub-harmonically related signals to the carrier frequency. For example, as shown in FIG. 2, the fifth harmonic frequency of the second LO signal 22, generated by the divide-by-four circuit 23, is the same frequency as the desired carrier frequency. Thus, the leakage of the second LO signal 22 to the antenna port (not shown) will produce the DC offset at the mixer 24 output. This simple observation shows that all harmonics or sub-harmonics of LO signals should be different from the carrier signal, to avoid producing the DC offset.
Since the signal isolation is not perfect in any RF front-end implementation, spectral leakage occurs through electromagnetic radiation, substrate-coupling, and parasitic coupling. In the related art implementations, this spectral coupling is compounded by harmonic components of LO signals having the same frequency as the carrier frequency, thereby producing the DC offset component at the down-conversion mixer output. This occurs in the related art implementations because the designs use a divide-by-N circuit to generate each of the mixing frequencies, from a voltage controlled oscillator (VCO) producing a signal at the RF carrier frequency. Since N is an integer in these designs, a harmonic having the same frequency as the RF carrier being mixed will necessarily be produced by the down-converter.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.